1. Field of the Invention
The invention relates to safety syringes in general and to an improved locking mechanism for safety syringes in particular.
2. Prior Art
In one form of safety syringe, the sheath is advanced by operation of the plunger. Examples of these safety syringes can be found in U.S. Pat. Nos. 5,720,727 and 5,460,611, which are both hereby incorporated by reference in their entirety. Syringes of this type offer excellent protection for the health care worker from accidental needle sticks; however, an additional concern is the reuse of the needles by intravenous drug users. Reuse of needles is a significant contributing factor to the spread of blood born diseases like AIDS and hepatitis. Many prior art safety syringes have locking mechanisms which are designed to secure the syringe in the safe position. However, most of these locking systems are designed to prevent the accidental return of the needle to an exposed condition. They are not designed to prevent a determined drug addict from overriding the locking mechanisms in order to reuse the needle for additional injections. While no lock or locking system can ever be completely invincible, a safety syringe meeting the following objectives is desired.
It is an object of the invention to provide a safety syringe capable of preventing accidental needle sticks.
It is another object of the invention to provide a safety syringe configured to be locked in its covered or safe position.
It is still another object of the invention to provide a safety syringe having safeguards to prevent its locking mechanism from being defeated.
It is yet another object of the invention to provide a safety syringe that cannot be used more than once.
The invention comprises a safety syringe having a barrel, a plunger slidably disposed in the barrel, and a needle extending from the needle end of the barrel. The plunger has a washer end configured to create a substantially fluid tight seal with the barrel sides. A sheath is slidably disposed over the needle. The sheath has an exposed position in which the sharp end of the needle is exposed and a covered position in which the sharp end of the needle is enclosed within the sheath. The sheath, or its tip end, is preferably hypodermically insertable. The sheath is moved from its is exposed position to its covered position by operation of the plunger. One or more arm, directly or indirectly, connects the sheath to the plunger so that when the plunger is fully depressed, the sheath will advance from the exposed position to the covered position.
There are several devices employed by the invention that lock the sheath in the covered position, either by preventing the sheath from being removed or by preventing it from being returned to the exposed position. A bar extends at least part of the way across the interior of the barrel. The arm or arms contain a pair of teeth that are designed to engage this bar. The leading edge of the first tooth is beveled to allow it to pass over the bar when the first tooth meets the bar as the arm and sheath are being advanced. The trailing edge of the first tooth is flat and will not allow the first tooth to ride back over the bar. This will prevent the sheath from backing up once the first tooth has passed the bar. The sheath, the arm, the bar, and the first tooth should be sized so that the sheath will be in the covered position by the time the first tooth passes the bar. The leading edge of the second tooth also has a flat surface. This will prevent the second tooth from passing over the bar. Thus, once the first tooth has passed the bar, the bar will be locked between the first tooth and the second tooth. While the first tooth prevents the sheath from backing up, the second tooth prevents the sheath from being removed by pulling it off over the sharp end of the needle, thus locking the sheath in the covered position.
In the preferred embodiment, there is a diaphragm that closes the needle end of the barrel. The diaphragm is preferably made of rubber and is configured to create a substantially fluid tight seal with the barrel sides. The diaphragm is provided with a central aperture which is slidably positioned around the needle so that when the plunger is fully depressed, the plunger will move the diaphragm forward along the needle. The arm or arms should be positioned so that they will be advanced by the diaphragm, and thus move the sheath into its covered position.
In a preferred embodiment, the diaphragm has a plunger side facing the plunger and a needle side facing the needle. The needle side of the diaphragm should contain one or more grooves. The plunger ends of the arms each have an enlarged head which are sized to fit into and engage the groove or grooves in the diaphragm. The enlarged heads preferably have a beveled leading edge to allow their insertion into the groove and a wide trailing edge to resist their extraction. The engagement of the arms with the diaphragm will resist extraction of the sheath over the sharp end of the needle, in combination with the second tooth""s engagement with the bar.
In another preferred embodiment, the plunger and the barrel are designed to lockingly engage one another when the plunger is fully depressed, preventing the plunger from being extracted. This will secure the plunger in its fully depressed position adjacent to the diaphragm. With the plunger locked there, it will prevent the diaphragm and the sheath from being pushed back toward the plunger end of the barrel. Together with the locking engagement between the trailing edge of the first tooth and the bar, this will prevent the sheath from being pushed back into the exposed position.
The plunger is preferably sized so that the entire plunger is contained within the barrel when the plunger is fully depressed. This will make the plunger more difficult to engage when it is fully depressed, and thus more difficult to forcibly extract when it is locked in place.